1. Field of the Invention
The harvesting of Radio Frequency (RF) energy is extremely important for Radio Frequency Identification (RFID), security monitoring and remote sensing. Numerous uses such as some harvesting devices may be tuned to a specific frequency (RFID), while others take advantage of the ambient energy (remote sensing) and others simply sense frequencies for security purposes.
2. Description of the Prior Art
Recharging devices using an RF electromagnetic field radiated into free space have been described. U.S. Pat. No. 6,127,799 entitled, “Method and Apparatus for Wireless Powering and Recharging”. This patent discloses a battery charging arrangement in which a rechargeable charge storage device is placed in an RF or microwave radiation field. In one aspect of the invention, a charge storage device is charged by exposing the charge storage device to an RF electromagnetic field radiated into free space. The charge storage device includes one or more antennae disposed on the device and adapted to receive the radiated RF electromagnetic field. One or more rectifiers are connected to the antennae for rectifying the received RF electromagnetic field into a DC output current. The DC output current produced by the rectifier is used to charge the charge storage device.
The antenna may be one or more dipole antennae which may be combined to form at least two subsets of dipole antenna element arrays, wherein one subset may be oriented at an acute or a right angle with respect to at least one other subset. The antennae or dipole antennae may be placed on more than one outside surface of the charge storage device which enclose an acute or a right angle with respect to each other. To efficiently couple the antenna array to the radiation field, each of the antenna elements of the array preferably has a length of approximately λ/2, wherein λ is the free space wavelength of the RF radiation. If more than one dipole is used, the spacing between adjacent antenna dipoles is preferably also λ/2. The number of dipoles determines the power conversion efficiency of the antenna array.
Resonant antennae such as dipole antennae are more efficient for a given length of wire, but are restricted to narrow bandwidths. Most present day users of antennae for communication purposes desire an antenna that can operate on multiple frequencies without requiring antenna tuning or matching devices. U.S. Pat. No. 5,111,213 entitled, “Broadband Antenna” discloses a broadband HF antenna in which the entire length (height) of the antenna radiates radio frequency energy. The insertion resistor is introduced at the top of the antenna or on the inside surface of the antenna radiating element. The radio frequency antenna current travels through the insertion resistor and continues for an appreciable electrical distance in a coaxial cable and/or other passive components. Thus, the invention provides greater usable electrical radiating element length for a given physical antenna length (height).
U.S. patent application Ser. No. 10/624,051 by Mickle et al entitled, “Energy Harvesting Circuits and Associated Methods” discloses an energy harvesting circuit that has an inherently tuned antenna, with at least portions of the energy harvesting circuit structured to provide regenerative feedback into the antenna to thereby establish an effective antenna area substantially greater than the physical area.
A problem with the prior art is that the energy harvesting circuits cannot efficiently harvest RF energy at frequencies outside the design specific frequency of the antenna. For example, in U.S. Pat. No. 6,127,799, the disclosed dipoles that make up the antenna array are fixed to absorb RF energy at a specific frequency range. The purpose of the array is to increase the efficiency of the power conversion and not to increase the bandwidth of the RF energy to be absorbed.
However, the antenna for efficient energy harvesting requires characteristics that are different from those of a communications antenna.
What is needed is antenna and forms of the antenna(e) that are specifically designed for energy harvesting so as to maximize the harvesting of RF energy.
There is a need for energy harvesting circuits that can harvest RF energy from a broad RF spectrum so as to maximize the harvesting of RF energy.
Also, there is a need for energy harvesting circuits that can harvest RF energy from a broad RF spectrum by having an antenna that can be broadly tuned to broadcast frequencies (or other ambient frequencies) so as to maximize the harvesting of RF energy.